Adhesion between glass and glass has hitherto been effected with a photocurable organic adhesive. The organic adhesive is easy to cure and is highly transparent but, in turn, insufficient in moisture resistance or heat resistance and undergoes large shrinkage on curing. Therefore, the conventional organic adhesive is lacking in adhesion reliability under high temperature and high humidity conditions.
On the other hand, cold curing liquid silicone rubbers are known as adhesive for glass. Since the adhesives of this type are cured through condensation reaction, it takes much time for them to cure when applied on a wide area, which has limited the range of their application. In order to solve this problem, a heat curing liquid silicone rubber which is cured taking advantage of addition reaction has been developed. However, this type of adhesive cannot be heated at a high temperature enough for curing if applied on parts of electronic equipments. Further, it is liable to have cracks in heat cycle testing. From these considerations as well as from the standpoint of energy saving, range of application of the heat curing silicone rubber is thus limited. It has been, therefore, keenly demanded to develop a photocurable silicone rubber adhesive.
There are several reports in the literature about photocurable silicone compositions. For example, U.S. Pat. No. 3,726,710 describes that a vinyl-containing polyorganosiloxane having incorporated therein a photo reaction initiator is curable on irradiation with highly intense ultraviolet rays. However, the composition disclosed is for use in the production of silicone release paper and fit for curing of a thin film but is of no use as adhesive for glass because it exhibits no adhesiveness to glass and considerably reduced curability with light transmitted by glass.
U.S. Pat. No. 3,816,282 discloses a composition comprising a mercapto-containing polyorganosiloxane, polymethylvinylsiloxane, and an organic peroxide of various kinds. This composition has a serious disadvantage of poor preservation stability at room temperature, readily undergoing gelation.
Japanese Patent Application (OPI) NO. 125123/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") describes a composition comprising a vinyl-containing polyorganosiloxane; a polyorganohydrogenosiloxane, and a photosensitizer, and Japanese Patent Application (OPI) No. 109852/82 describes a composition comprising a vinyl-containing polyorganosiloxane and an organic peroxide. Both of these compositions suffer from shortage of adhesion to glass and curability with light transmitted by glass and cannot be, therefore, used as an adhesive for glass.
Apart from these photocurable silicone compositions, Japanese Patent Publication No. 44778/80 proposes a self-bonding heat curable silicone rubber composition comprising an polyorganosiloxane having a viscosity of at least 100,000 cP at 25.degree. C., an organic peroxide as a curing catalyst, and an isocyanurate derivative. The composition involves the similar problems as associated with the aforesaid heat curing liquid silicone rubber, as it cures and adheres on heating. In addition, the high viscosity of the polyorganosiloxane permits of no use as an adhesive.
Thus, there has not yet been developed a satisfactory adhesive for glass which would meet the requirements of heat resistance, moisture resistance, and rapid curability for use, for example, in optical parts of electronic equipments.